STRUCTURE AND DEVELOPMENT OF BRACTEAL NECTARY GLANDS IN APHELANDRA (ACANTHACEAE)

A survey of bracteal (extrafloral) nectaries in species of Aphelandra (Acanthaceae) reveals substantial diversity. Each bracteal nectary is an aggregate of individual glands that vary in number, size, and struc ture among species. Glands contain three cell layers: a palisade-like secretory cell layer, a one-to-many-celled intermediate layer with thi ckened cell walls, and a foot layer Members of the A. pulcherrima comp lex have one of two distinct gland types: relatively small glands with a single-celled intermediate layer or larger glands that have a multi cellular intermediate layer. Nectaries composed of small glands are pa tches of many (>50) glands, whereas those composed of large glands are patches of <10 glands. Four outgroup species have bracteal nectaries of numerous small glands with pluricellular intermediate layers. Gland s of all three types are initiated as single enlarged protodermal cell s, and all undergo similar early periclinal divisions; the large-gland type shows greater subsequent enlargement with many more anticlinal d ivisions. The bracteal nectar glands are interpreted to be homologous with simpler glandular trichomes, and mark a monophyletic lineage with in Aphelandra. Comparisons with outgroup species show that both nectar y types in the A. pulcherrima complex have diverged from an ancestral condition of numerous small glands with pluricellular intermediate lay ers. Use of the ontogenetic criterion to polarize gland type within th e A. pulcherrima complex would yield erroneous results because evoluti on has apparently involved a developmental truncation with loss of cel l divisions in the intermediate layer of small glands. Comparable nect ar glands in more distant taxa are interpreted as remarkable cases of convergent evolution, perhaps from similar trichome precursors.